gear ratios paired to drivetrains wasn't yet standardized in 86 so it could have pretty much anything (3.31, 3.55, 3.73, 4.10). easiest way to check is to jack up one rear tire and rotate it around exactly twice while counting the number of times the driveshaft rotates. for example: 3 and a half times is 3.55 gears
the 3.4 is a better built engine (it's essentially just a newer version of the 2.8), but if there's nothing horribly wrong inside your 2.8, I personally probably wouldn't put forth the effort to do the swap unless I was also swapping in the EFI.
to go through that much gas I'd guess that the tank leaks. could just be the pump seal, but could also be rust. hard to say without looking.
the 44 rear axle technically wasn't available in 86, but anything could happen in 3 decades so it's best to look and see.
drop your vin into the registry section: https://comancheclub.com/forum/29-1986/